This invention relates to arrangements for retaining magnets radially in position in permanent magnet rotors for electric machines which operate at high speed.
Conventional permanent magnet rotors have a circumferential spaced array of pole pieces which are made of magnetic material and are shaped to achieve optimum magnetic performance along with permanent magnets which are retained in the spaces between the pole pieces by slot wedges having flat outer surfaces that span the distance between the poles. The slot wedges are made of nonmagnetic material to avoid shunting magnetic flux and have edges which are received in recesses in the pole pieces to restrain them and the magnets beneath them from radial displacement during rotation of the rotor. When the rotor is rotated at high speed, however, the slot wedges may not have sufficient rigidity to maintain the magnets in fixed positions in the radial direction.
The Lavin et al. U.S. Pat. No. 3,169,203 discloses a square wave pulse generator having a permanent magnet rotor in which T-shaped permanent magnets are mounted on a central hexagonal core and retained in position radially by soft iron shoes formed from laminations which are held against the outer surface of the magnets by lateral projections which are embedded in epoxy resin members disposed between the magnets. Stainless steel plates at opposite ends of the rotor are joined by rivets which extend through the epoxy resin members.
The Patel U.S. Pat. Nos. 4,633,113 and 4,674,178 disclose a permanent magnet rotor having a hexagonal hub and permanent magnets which are adhesively bonded to the hub surfaces, the spaces between the magnets being filled by epoxy material which is then ground off so that the magnets and intervening filler material have a common cylindrical surface. An outer layer is provided by winding a fiber such as carbon fiber in a curable resin such as an epoxy resin circumferentially about the cylindrical surface after which the resulting layer is cured and ground to provide a smooth cylindrical surface.
The Brown U.S. Pat. Nos. 4,729,160 and 4,930,201 disclose a permanent magnet rotor having an array of permanent magnets adhesively bonded to a hub and a sleeve surrounding the magnets and the intervening filler pieces which is made of a composite material consisting of high strength, high modulus fibers such as fiberglass, graphite, boron or the like in an epoxy or other plastic matrix.
Accordingly, it is an object of the present invention to provide a magnet retention arrangement for permanent magnet rotors operating at high speed which overcomes disadvantages of the prior art.
Another object of the invention is to provide a permanent magnet retention arrangement for high speed rotors which has a smooth cylindrical peripheral surface to reduce windage losses while securely retaining the magnets in position.
These and other objects of the invention are attained by providing a permanent magnet rotor having a rim member on which an array of angularly spaced pole pieces are retained by mechanical connections such as screws, and an array of magnets disposed between the pole pieces are held in position radially by peripheral wedges which have an interlocking connection, such as a tongue and groove connection, with the adjacent pole pieces and the wedges and pole pieces forming a smooth outer common cylindrical surface, along with a fiber wrap surrounding the rotor assembly.
In a preferred embodiment the pole pieces may be assembled from laminated magnetic material or be constituted by electrically isolated pressed powder to form a solid member and permanent magnets inserted in the spaces between the pole pieces are radially retained by slot wedges which are flat bars having edges with slots shaped to receive projections from the adjacent portions of the pole pieces. The pole pieces and the slot wedges are machined to provide a smooth cylindrical common outer surface which is covered by a composite wrap of fiber-reinforced polymer material to maintain the radial rigidity of rotor structure in response to centrifugal forces acting on the magnets during high speed operation. With this arrangement, the irregular peripheral shape of conventional permanent magnet rotors is replaced by a smooth cylindrical surface to reduce windage losses and the secure retention of the magnets by the composite outer wrap permits higher rotational speed of the rotor.